The present invention relates to an epoxy resin composition for fiber reinforced plastics (abbreviated to FRP in this specification), a prepreg that is an intermediate material made up of a combination of an epoxy resin composition and reinforcing fibers, and a tubular molded article obtained by the use thereof.
This application is based on a patent application in Japan (Japanese Patent Application No. Hei 9-74794) and the contents described in that Japanese application are taken as part of this description.
Since epoxy resins after curing are excellent in mechanical properties, electrical properties, and adhesive properties, they are widely used in the fields, for example, of sealants for electronic materials, paints, coating materials, and adhesives. Further, epoxy resins are important as matrix resins for FRP. Particularly, when carbon fibers are used as reinforcing fibers for FRP, epoxy resins are preferably used since they are excellent in adhesion to carbon fibers. FRP molded articles made up of carbon fibers and epoxy resins are used in wide range of applications ranging from general-purpose use in fishing rods, golf club shafts, etc. to the use in airplanes.
The method for molding FRP molded articles from reinforcing fibers, such as carbon fibers, and a matrix resin, such as an epoxy resin, includes several methods. If carbon fibers are used as reinforcing fibers, a method wherein an intermediate material called a prepreg prepared by impregnating in advance the reinforcing fibers with a resin is used for the molding of an FRP molded article is most widely used. The matrix resin used in this prepreg is required to be excellent, for example, in adhesion to the reinforcing fibers and mechanical properties after the molding. Further, the prepreg is required to have suitable tack in order to make the handling good. Since epoxy resins can be made to exhibit relatively easily these properties in a well balanced manner, they are widely used as a matrix resin for prepregs.
The major application of these FRP molded articles includes tubular molded articles, such as fishing rods and golf club shafts. Such tubular molded articles are required to have, as important properties, flexural strength in the longitudinal direction and crushing strength in the diametrical direction.
In order to enhance the flexural strength of tubular molded articles in the longitudinal direction, it is already known that it is effective to arrange carbon fibers along the length. Further, in order to improve the crushing strength of tubular molded article in the diametrical direction, it is already known that it is effective to arrange carbon fibers circumferentially. However, when a tubular molded article is reinforced longitudinally and circumferentially at the same time, the tubular molded article inevitably increases in weight. An increase in weight of a tubular molded article runs counter to the recent trend wherein fishing rods and golf club shafts are made light in weight. Accordingly, there have been attempted to improve matrix resins to reduce the circumferential reinforcement of tubular molded articles. However, matrix resins that can make tubular molded articles light in weight are not now available.
The present invention aims at providing an epoxy resin composition for FRP in order to obtain tubular molded articles improved in flexural strength in the longitudinal direction and crushing strength in the diametrical direction and a prepreg made of a combination of that epoxy resin composition for FRP and reinforcing fibers.
The first subject matter of the present invention resides in an epoxy resin composition for FRP, characterized in that it comprises (A) a bisphenol A-type epoxy resin (hereinafter referred to as component (A)), (B) an epoxy resin having oxazolidone rings represented by (hereinafter referred to as component (B)), and (C) a curing agent (hereinafter referred to as component (C)), the component (B) is an epoxy resin having a structure represented by the following formula (1): 
wherein R represents a hydrogen atom or a methyl group, and the viscosity measured by the below-described method for measuring viscosities is 100 to 5,000 poises.
Use is made of a dynamic viscoelasticity measuring instrument, the uncured epoxy resin composition for FRP whose viscosity will be measured is filled between two disk plates 25 mm in diameter that are positioned with a space of 0.5 mm between them, one of the disk plates is rotated at a shear rate of 10 radians/sec, and the viscosity of the epoxy resin is measured under conditions in which the atmospheric temperature for the measurement is 60xc2x0 C.
The second subject matter resides in an epoxy resin composition for FRP, characterized in that it comprises a component (A), a component (B), and a component (C), and (D) a thermoplastic resin that can be dissolved in a mixture of the component (A) and the component (B) (hereinafter referred to as component (D)) and has a viscosity of 100 to 5,000 poises measured by the above method.
Further, the third subject matter resides in a prepreg comprising a sheet of reinforcing fibers impregnated with the above epoxy resin composition for FRP.
Further the fourth subject matter resides in a tubular molded article having a plurality of FRP layers, characterized in that the matrix resin composition used in at least one of the FRP layers is the above epoxy resin composition for FRP.
And the fifth subject matter resides in an epoxy resin composition for FRP, characterized in that when the epoxy resin composition for FRP is molded into a tubular product, the crushing strength is 200 N or more.
In the present invention, xe2x80x9ccrushing strengthxe2x80x9d means the crushing strength, measured by the below-described method, of a tubular molded article that has an inner diameter of 10 mm, an outer diameter of 12 mm, and a volume content of fibers of 60+1% and is prepared by impregnating carbon fibers having an elastic modulus of 220 to 250 GPa with the epoxy resin composition for FRP to make unidirectional prepregs wherein the carbon fiber areal weight is 150 g/m2 and the content of the epoxy resin for FRP is 31% by weight and laminating the unidirectional prepregs so that the directions of the fibers may be +45xc2x0/xe2x88x9245xc2x0/+45xc2x0/xe2x88x9245xc2x0/0xc2x0/0xc2x0/0xc2x0.
The above-described tubular molded article is cut into a length of 10 mm to obtain a test piece. Using an indenter, a load is exerted on the test piece and the maximum load at which the test piece is broken when the indenter is moved at a rate of travel of 5 mm/min is measured and is designated as the crushing strength.
Further, the sixth subject matter resides in an epoxy resin composition for FRP, characterized in that when the epoxy resin composition for FRP is made into a unidirectional laminate, the flexural strength in a direction of 90xc2x0 is 110 MPa or more.
In the present invention, xe2x80x9cflexural strength in a direction of 90xc2x0xe2x80x9d means the flexural strength of 90xc2x0, measured by the below-described method, of the unidirectional laminate that is prepared by impregnating carbon fibers having an elastic modulus of 220 to 250 GPa with the epoxy resin composition for FRP to make unidirectional prepregs wherein the carbon fiber areal weight is 150 g/m2 and the content of the epoxy resin for FRP is 31% by weight and laminating fifteen unidirectional prepregs thus made (2 mm in thickness) so that the directions of the fibers may be 0 degrees.
The above-described unidirectional laminate is cut to obtain a test piece having a length of 60 mm in a direction of 90xc2x0 to the direction of the fibers and a width of 10 mm. The maximum load when the test piece is broken is measured under conditions wherein the distance between the supports is 32 mm, the diameter of the tip of the indenter is 3.2 mm, and the rate of travel of the indenter is 2 mm/min and the flexural strength is calculated.
As the component (A) of the epoxy resin composition for FRP of the present invention, those which are generally on the market can be used. As the component (A), those which when used in the epoxy resin composition for FRP bring its viscosity to the range described later may be used. The epoxy equivalent, the molecular weight, and the state at normal temperatures of the component (A) are not particularly restricted. Herein, xe2x80x9cepoxy equivalentxe2x80x9d in the present invention means the number of grams of the resin containing epoxy groups in an amount of 1 gram equivalent. As the component (A), a mixture of a bisphenol A-type epoxy resin having an epoxy equivalent of 300 or less that is a liquid or a semisolid at normal temperatures and a bisphenol A-type epoxy resin having an epoxy equivalent of 400 or more that is a solid at normal temperatures is preferably used because the strength in a direction of 90xc2x0 to the direction of fibers can be exhibited. Further, particularly preferably, the bisphenol A-type epoxy resin having an epoxy resin equivalent of 300 or less is contained in an amount of 25 to 65% by weight in the component (A).
As typical ones of the bisphenol A-type epoxy resin having an epoxy equivalent of 300 or less that is a liquid or a semisolid at normal temperatures, EPIKOTE 828 and EPIKOTE 834 manufactured by Yuka Shell Epoxy K. K. can be mentioned by way of example.
As representative examples of the bisphenol A-type epoxy resin having an epoxy equivalent of 400 or more that is a solid at normal temperatures, EPIKOTE 1001, EPIKOTE 1002, EPIKOTE 1004, EPIKOTE 1007, and EPIKOTE 1009 can be mentioned by way of example.
As the component (B) of the epoxy resin composition for FRP of the present invention, an epoxy resin having oxazolidone rings represented by the below-shown formula (1) is used. This component (B) is an essential component for the purpose of the present invention for obtaining a tubular molded article that exhibits a high crushing strength. An epoxy resin composition for FRP that does not contain this component (B) can provide neither a tubular molded article having both a high crushing strength and a high flexural strength nor a prepreg good in handleability. Further it is also necessary to use an epoxy resin containing oxazolidone rings and epoxy groups in the molecule. A mixture of a compound containing oxazolidone rings and a compound containing epoxy groups can provide neither a tubular molded article having both a high crushing strength and a high flexural strength nor a prepreg good in handleability. 
wherein R represents a hydrogen atom or a methyl group.
As the component (B), an epoxy resin having a structure represented by the below-shown formula (2) is particularly preferable. This component (B) can be synthesized by a method disclosed in Japanese Patent Application, First Publication No. Hei 5-43655 wherein an epoxy resin and an isocyanate compound are reacted in the presence of an oxazolidone ring forming catalyst. Further, as commercially available epoxy resins that can be used as the component (B), XAC4151 and XAC4152 manufactured by Asahi-Ciba Limited can be mentioned by way of example. 
wherein R""s each independently represent a hydrogen atom or a methyl group, R1 to R4 each independently represent a halogen atom, a hydrogen atom, or an alkyl group having 1 to 4 carbon atoms, R5 to R8 each independently represent a hydrogen atom or an alkyl group having 1 to 4 carbon atoms, and R9 has the below-shown formula (3) or the below-shown formula (4). 
wherein Rxe2x80x21 to Rxe2x80x24 each independently represent a hydrogen atom or an alkyl group having 1 to 4 carbon atoms. 
wherein Rxe2x80x21 to Rxe2x80x28 each independently represent a hydrogen atom or an alkyl group having 1 to 4 carbon atoms and Rxe2x80x29 represents a single bond, xe2x80x94CH2xe2x80x94, xe2x80x94C(CH3)2xe2x80x94, xe2x80x94SO2xe2x80x94, xe2x80x94SOxe2x80x94, xe2x80x94Sxe2x80x94, or xe2x80x94Oxe2x80x94.
As the component (C) of the epoxy resin composition for FRP of the present invention, any curing agent can be used that is used as a curing agent for usual epoxy resin compositions. As typical curing agents, dicyandiamides, ureacompounds, amine compounds, acid anhydrides, imidazole compounds, phenol compounds, etc. can be mentioned by way of example. Among others, a combination of a dicyandiamide and a urea series compound is particularly preferable because the curability of the epoxy resin composition for FRP and the balance among the physical properties of the molded article obtained after the curing are good.
To the epoxy resin composition for FRP of the present invention, the component (D) can be added. By adding the component (D) to the epoxy resin composition for FRP, the handleability of the prepreg is further improved and the properties of the obtainable tubular molded article, such as the crushing strength, are improved and stabilized.
As the component (D) of the epoxy resin composition for FRP of the present invention, a thermoplastic resin that dissolves uniformly in a mixture of the component (A) with the component (B) is used. As the component (D), phenoxy resins and polyvinylformal resins are particularly preferably used. The phenoxy resins are not particularly restricted and those generally on the market can be used. For example, PHENOTOHTO YP-50 and PKHP-200 manufactured by Tohto Kasei Co., Ltd. etc. can be mentioned as typical examples thereof. As the polyvinylformal resin (hereinafter referred to as PVF), a resin containing 60% by weight or more of a vinyl formal moiety with the rest made up of vinylalcohol, vinylacetate, or the like is used. VINYLEC manufactured by Chisso Corporation can be mentioned as commercially available typical examples thereof.
The epoxy resin composition for FRP of the present invention is required to have a viscosity of 100 to 5,000 poises measured by the below-described method for measuring viscosities. If the viscosity of the epoxy resin composition for FRP is less than 100 poises at 60xc2x0 C., the tack becomes too strong or the resin flow at the time of molding becomes too great, making it impossible to obtain the intended properties after the molding, which is unpreferable. Furthermore, if the viscosity of the epoxy resin composition for FRP is over 5,000 poises, the impregnation with the resin at the time of formation of a prepreg becomes insufficient, the tack is lost to too great an extent, or the prepreg becomes hard, making it impossible to obtain the intended properties after the molding, which is unpreferable. A more preferable range is from 300 to 3,000 poises.
The viscosity in the present invention is the viscosity measured by using a dynamic viscoelasticity measuring instrument, such as a dynamic viscoelasticity measuring instrument RDA-700 manufactured by Rheometric Scientific F. E. Ltd. in such a manner that the uncured epoxy resin composition for FRP whose viscosity will be measured is filled between two disk plates which are 25 mm in diameter that are positioned with a space of 0.5 mm between them, one of the disk plates is rotated at a shear rate of 10 radians/sec, and the viscosity of the epoxy resin is measured under conditions in which the atmospheric temperature for the measurement is 60xc2x0 C.
The formulation ratio of the components in the epoxy resin composition for FRP of the present invention is not particularly restricted so long as the composition satisfies the above conditions including the viscosity conditions. If the component (D) is not contained, the formulation ratio [(A)/(B)] of the component (A) and the component (B) is preferably in the range of from 9/1 to 3/7 by weight. Further, particularly preferably, the component (B) accounts for 20 to 50% by weight of the mixture of the component (A) and the component (B).
Further, when the component (D) is contained, the formulation ratio [(A)/(B)] of the component (A) and the component (B) is preferably in the range of from 15/1 to 1/5 by weight and more preferably in the range of from 10/1 to 1/3 by weight. Further, most preferably, the component (B) accounts for 7 to 50% by weight of the mixture of the component (A) and the component (B).
When only a phenoxy resin is used as the component (D) of the epoxy resin composition for FRP of the present invention, the formulation ratio {(D)/[(A)+(B)]} of the components in the epoxy resin composition for FRP is preferably in the range of from 1/100 to 30/100 by weight and more preferably in the range of from 2/100 to 20/100 by weight. Further, when only a polyvinylformal resin is used as the component (D), the formulation ratio {(D)/[(A)+(B)]} of the components in the epoxy resin composition for FRP is preferably in the range of from 1/100 to 20/100 and more preferably in the range of from 1/100 to 10/100.
To the epoxy resin composition for FRP of the present invention, another epoxy resin (E) (hereinafter referred to as component (E)) can be added in the range wherein the above formulation ratios are satisfied. By adding the component (E) to the epoxy resin composition for FRP, the properties of the tubular molded article, such as the heat resistance, can further be improved.
As the component (E),. for example, bisphenol F-type, bisphenol S-type, glycidylamine-type, aminophenol-type, phenolic novolak- type, and cresol novolak-type epoxy resins, or aliphatic, cycloaliphatic, and other epoxy resins can be mentioned. Among others, phenolic novolak-type epoxy resins having a softening point of 60xc2x0 C. or more are particularly preferable because they less adversely affect the balance of the entire properties of the epoxy resin composition for FRP. As typical examples of phenolic novolak-type epoxy resins having a softening point of 60xc2x0 C. or more, EPICLON N-770 and N-775 manufactured by Dainippon Ink and Chemicals Inc. can be mentioned.
When the component (E) is added to the epoxy resin composition for FRP, the component (E) is contained in an amount of 3 to 20% by weight based on the total of the epoxy resin components consisting of the component (A), the component (B) and the component (E). Further, more preferably, the total amount of the component (B) added and the amount of the component (E) added is 20 to 50% by weight based on the total of the epoxy resin components consisting of the component (A), the component (B), and the component (E), and the composition ratio of the component (B) to the composition (E) is in the range of from 1/3 to 4/1.
As the epoxy resin composition for FRP of the present invention, those that are excellent in toughness in addition to the above conditions are more preferable. Specifically, an epoxy resin composition for FRP whose GIC (critical strain energy release rate) is 400 J/m2 or more is particularly preferable. Herein, GIC can be measured by the compact tension method described in ASTM-E399.
A sheet of reinforcing fibers is impregnated with the epoxy resin composition for FRP of the present invention to make them integrated, thereby forming an intermediate material called a prepreg.
The reinforcing fibers used in the present invention are not particularly restricted and use is made of carbon fibers, glass fibers, aramid fibers, boron fibers, steel fibers, etc. singly or in combination. Among others, carbon fibers are particularly preferably used since the mechanical properties after molding are good.
As the carbon fibers, any of PAN precursor carbon fibers and pitch precursor carbon fibers can be used. Further, as the carbon fibers, various carbon fibers different in strength and elastic modulus can be selected and used according to the purpose.
Further, as the prepreg of the present invention, for example, a unidirectional prepreg wherein reinforcing fibers are arranged in one direction, a fabric wherein reinforcing fibers are woven, a nonwoven fabric of reinforcing fibers, or a prepreg prepared by impregnating a tow of reinforcing fibers directly with the epoxy resin composition for FRP of the present invention can be mentioned.
In the present invention, the sheetlike product wherein reinforcing fibers are arranged in one direction, the fabric wherein reinforcing fibers are woven, the nonwoven fabric made up of reinforcing fibers, the tow of reinforcing fibers, and the like are simply referred to as a sheet of reinforcing fibers.
The method for impregnating reinforcing fibers with the epoxy resin composition for FRP of the present invention is not particularly restricted. As the method for impregnating reinforcing fibers with the epoxy resin composition for FRP, a method wherein a sheet of reinforcing fibers is impregnated with the resin from both surfaces thereof is preferred over a method wherein a sheet of reinforcing fibers is impregnated with the resin from one surface thereof because, in the former case, when the sheet is molded into the tubular molded article intended by the present invention, the crushing strength and the flexural strength in a direction of 90xc2x0 are improved.
When the epoxy resin composition for FRP of the present invention is made into a prepreg, the prepreg has suitable tack and flexibility and is good in the balance between stability with time and curability. In particular, when the epoxy resin composition for FRP of the present invention is made into a unidirectional prepreg of carbon fibers, such an effect is brought about that the flexural strength in a direction of 90xc2x0 to the axes of the fibers of the molded article is considerably improved. As a result, the tubular molded article obtained by using the prepreg is improved considerably in crushing strength and flexural strength. Such an effect is difficult to obtain using conventional matrix resins and thus reflects the excellent performance of the epoxy resin composition of the present invention.
In particular, when a prepreg having a flexural strength of 125 MPa or more in a direction of 90xc2x0 to the axes of the fibers is used, the crushing strength of the obtained tubular molded article is remarkably improved. Thus, the crushing strength is greatly affected by flexural strength in a direction of 90xc2x0 to the axes of the fibers. Further, the effect of the elastic modulus of the carbon fibers used is also great and when carbon fibers having a high elastic modulus are used, generally a favorable crushing strength is readily be obtained.
Accordingly, when carbon fibers having a high elastic modulus are used, a good crushing strength is obtained even when the flexural strength in a direction of 90xc2x0 to the axes of the carbon fibers is a little low. In particular, a good crushing strength is obtained in the case in which the flexural strength in a direction of 90xc2x0 to the axes of the fibers and the elastic modulus of the carbon fibers used satisfy the following relationship:
flexural strength in a direction of 90xc2x0 [MPa]xe2x89xa72,500/the elastic modulus of the carbon fibers [GPa]
On the other hand, the lower the resin content in the prepreg is, the better it is, since the tubular molded article is made lighter in weight. However, a decrease in the resin content is apt to result in lowering of the flexural strength in a direction of 90xc2x0 to the axes of the fibers. Particularly, in the case in which the resin content is less than 25% by weight, the tendency for the lowering thereof is increased. Therefore, when a conventional epoxy resin composition was used as a matrix resin, it was difficult to secure sufficient crushing strength with the resin content of the prepreg lowered to make the tubular molded article light in weight. However, when the epoxy resin composition of the present invention is used as a matrix resin, even if the resin content of the prepreg is brought to less than 25% by weight, the obtained tubular molded article exhibits a crushing strength improved more than that of the conventional one. Accordingly, the tubular molded article can be made light in weight with the properties including crushing strength satisfactorily maintained.
Particularly, when a tubular molded article is molded using a prepreg in which the flexural strength in a direction of 90xc2x0 to the axes of the fibers, and the resin content of the prepreg and the elastic modulus of the carbon fibers satisfy the below-given relationship, the obtained tubular molded article is good in the balance between the effect of weight reduction and the physical properties including the crushing strength:
flexural strength in a direction of 90xc2x0 [MPa]xe2x89xa7X/the elastic modulus of the carbon fibers [GPa]
wherein X represents 100,000xc3x97the resin weight fraction of the prepreg
Next, the tubular molded article made of FRP in which the above-described epoxy resin composition is used is described.
The tubular molded article of the present invention is a tubular molded article excellent in crushing strength and flexural strength and is obtained by using the above-described epoxy resin composition for FRP for the matrix resin of at least one FRP layer out of a plurality of FRP layers.
This tubular molded article is generally obtained by winding layers of prepregs around a mandrel and heating and pressing them. It is required that at least one of these layers consist of the prepreg which is the third mode of the present invention in order to obtain the desired properties. Further, if all of the layers consist of the prepregs of the present invention, excellent properties are obtained without any problems. However, since respective layers are generally allocated respective roles, it is not necessarily required that all the layers consist of the prepregs of the present invention.
Further, it is possible to use layered prepregs made up of the prepregs of the present invention and conventional prepregs, which results in more preferable physical properties in many cases. Further, the tubular molded article can be obtained by a method wherein tow prepregs are wound to form layers. As the heating and pressing method used in molding the tubular molded article, compression molding in which a mold, such as a metal mold, is used, autoclave molding, vacuum bag molding, tape lapping molding, etc. can be mentioned by way of example, but the method is not necessarily limited to them.
The epoxy resin composition for FRP of the present invention is preferably such that when the epoxy resin composition is molded into the tubular molded article, the crushing strength of the tubular molded article is 200 N or more, the tubular molded article having an inner diameter of 10 mm, an outer diameter of 12 mm, and a volume content of fibers of 60+1% and being prepared by impregnating carbon fibers having an elastic modulus of 220 to 250 GPa with the epoxy resin composition for FRP to make unidirectional prepregs in which the carbon fiber areal weight is 150 g/m2 and the content of the epoxy resin for FRP is 31% by weight and laminating the unidirectional prepregs so that the directions of the fibers may be +45xc2x0/xe2x88x9245xc2x0/+45xc2x0/45xc2x0/0xc2x0/0xc2x0/0xc2x0. By defining the crushing strength of the above tubular molded article as being 200 N or more, the tubular molded article can be made light in weight by decreasing the number of the laminated FRP layers. The crushing strength of the tubular molded article is more preferably 240 N or more.
The epoxy resin composition for FRP of the present invention is preferably such that when carbon fibers having an elastic modulus of 220 to 250 GPa are impregnated with this epoxy resin composition for FRP to make unidirectional prepregs in which the carbon fiber areal weight is 150 g/m2 and the content of the epoxy resin for FRP is 31% by weight and fifteen unidirectional prepregs thus made are laminated so that the directions of the fibers may be zero degrees, thereby making a unidirectional laminate (2 mm in thickness), the flexural strength in a direction of 90xc2x0 is 110 MPa or more. Thus, by making the flexural strength of the above unidirectional laminate in a direction of 90xc2x0 to be 110 MPa or more, the weight of the tubular molded article can be made light by decreasing the number of the laminated layers of the FRP layers. More preferably, the flexural strength of the above unidirectional laminate in a direction of 90xc2x0 is 124 MPa or more and particularly preferably 140 MPa or more.